The present invention relates to the treatment of spent aqueous etchant solutions containing zirconium, nitric acid and hydrofluoric acid, from zirconium tube manufacturing processes. More particularly, the present invention relates to a series of treatment steps performed upon the etchant solution to recover zirconium and to restore the nitric acid to a concentration suitable for reuse in the etching process.
In the process for manufacturing nuclear fuel tubes, a nitric acid etching solution is employed. After processing this solution is found to contain approximately 25-30% nitric acid, 2-3% zirconium, 1-1.5% hydrofluoric acid and small amounts of other metals such as tin, iron, chrominum and nickel. For example, in a typical manufacturing process, as much as 3,000 gallons per week of spent etching solution might be produced. In some plants, the spent etching solution is treated as a waste by-product. However, if treated, it would be possible to recover significant amounts of zirconium and nitric acid for recycling of the acid to the fuel tube etching process.
Commercial processes for treating such a waste product involve evaporation and crystallization of the solution. Such evaporation processes have the following disadvantages. Firstly, high equipment costs are present due to the requirements for using chemically stable and heat resistant materials. Secondly, the process is not selective to zirconium and thus, further purification is required for reuse of zirconium. Thirdly, since a large volume of water should be evaporated, high energy costs are incurred. Fourthly, the process involves the loss of nitric acid since single stage evaporation produces vapor containing a significant amount of nitric acid. Fifthly, scale formation in the heat exchanger results in high maintenance cost.